With the recent increase in mobile stations such as PHS devices and mobile phones, there is an increasing social need for more effective use of radio wave frequency resources. An example of a communication method that addresses this need is the spatial multiplexing method.
The spatial multiplexing method is a communication method that involves the usage of an adaptive array apparatus that forms a different directivity pattern (called an adaptive array pattern) for each of a plurality of mobile stations. By doing this, the adaptive array apparatus is able to perform communication functions by multiplexing the transmission and reception signals of the plurality of mobile stations simultaneously on the same frequency.
An adaptive array apparatus includes a plurality of radio units made up of an antenna, a transmission unit, and a reception unit. And this apparatus forms directivity patterns (called array antenna patterns) for the entire antenna by adjusting the amplitudes and phases of the transmission signals and reception signals inputted to and outputted from each radio unit. More specifically, the array antenna patterns are formed by weighting the amplitudes and phases of the transmission signals and reception signals inputted to and outputted from each radio unit using a weight coefficient (also called a weight vector). The calculation of weight vectors is performed by a DSP (Digital Signal Processor) within the adaptive array apparatus.
When an adaptive array is used in a mobile phone system, the mobile phone unit has physical limitations such as size and possible number of antennas, making it impossible for the mobile phone to perform the function of controlling the directivity pattern. Therefore, it is the radio base station that forms the directivity patterns both at times of reception and times of transmission. In that case, the array antenna pattern formed by the radio base station at the time of transmission is same as the one that has been optimally formed at the time of reception.
However, even if the weight vector calculated at the time of reception is used at the time of transmission, it is not always the case that the same array antenna pattern will be formed for transmission and reception. This is due to the fact that, within each radio unit, the transfer characteristics of the transmission unit vary from those of the reception unit. These differences stem, for example, from the fact that the units are two physically separate circuits and there are variations between the characteristics of the circuit elements. In particular, this variation between the characteristics of the circuit elements come about in LNAs (Low Noise Amps) found in reception units and HPAs (High Power Amps) found in transmission units, these amps varying individually or undergoing temperature changes in certain usage environments. Due to these factors, variation comes about in the transfer characteristics such as phase rotation amounts and amplitude fluctuation amounts produced when a signal is passed through the transmission unit and the reception unit.
The differences between the transfer characteristics of reception units and transmission units have direct influences to errors in array antenna patterns at times of receiving and transmitting. For this reason, it is necessary that differences in transfer characteristics between transmission units and reception units be measured in order to perform calibration to compensate for those differences. An example of such a calibration method can be found in Japanese Laid-Open Patent Application No. H11-312917, titled “Array Antenna Apparatus”.
This array antenna includes, as additional equipment, a calibration-specific desired signal generation means, a calibration-specific interference signal generation means, an electrical power control means for controlling the electrical power of the calibration-specific interference signal generation means, a combining means for combining the calibration-specific desired signal with the calibration-specific interference signal whose electrical power is controlled, and a distribution means for distributing the combined signal to the various antennas, so as to compensate for the transfer characteristics within a reception system.
This additional equipment is required within the adaptive array apparatus of the prior art to assess the difference between transfer characteristics of transmission circuits and reception circuits in each separate radio unit, causing a problem wherein the scale of the circuits are increased. In other words, the problem with the prior art is that the scales of its circuits are increased in order to include a calibration-specific circuit that would not be required in regular communication.
In light of the above problems, an object of the present invention is to provide an adaptive array apparatus, a radio base station, and a mobile phone that reduce the size of the required circuitry and make corrections so that the same array antenna patterns are formed at times of receiving and times of transmitting.